DE102022201047A1 - Gas cell for enhanced light-matter interaction for integrated measurement systems - Google Patents

Abstract

The invention relates to a gas cell (1, 2) comprising an enclosed gas in a disc-shaped cavity, the wall of the cavity (11, 12, 21, 22) having at least one transparent entry window and one transparent exit window for light of a predetermined wavelength interval, and wherein the light is guided from an optical input (111, 2311) at the entry window to an optical output (111, 2311) at the exit window through a gas-filled hollow-core waveguide arrangement (13, 23), characterized in that the hollow-core Waveguide arrangement (13, 23) comprises light cages (131, 231) running parallel in sections.

Description

background

The invention relates to a gas cell comprising an enclosed gas in a housing, the housing having at least one transparent entry window and one transparent exit window for light of a predetermined wavelength interval, and the light passing from the entry window to the exit window through a hollow-core waveguide filled with the gas ("hollowcore waveguide", HCWG) arrangement.

Light-matter interaction has always been an essential basis of optical sensors. The identification of gas species based on absorption spectra is well known, especially for gases. Likewise, for example, gas compositions can be analyzed, impurities can be quantified using light scattering or chemical reactions can be observed, and measured variables that only indirectly affect a gas can be made visible and recorded through light-gas interaction, for example sound waves with a Schlieren method.

For some years, gas cells with alkali metal vapors have been considered as interesting candidates for integration into atomic measurement systems to exploit quantum effects. By means of optical fields, narrow-band absorption lines can be spectroscopy for the realization of optical frequency references. Atomic gases can be optically pumped and used as sensitive magnetic field probes or stimulated to form a distinctly narrow-band, transparent window on one of their absorption lines ("electromagnetically induced transparency", EIT), which can be used, among other things, as a very sensitive detection mechanism and for the storage of quantum information leaves.

For technical exploitation, it is important to develop systems that are as small and compact as possible - ideally fully integrable chip technology, for example based on glass/silicon - which also include miniaturized gas cells for providing the light-matter interaction. In order to remain compatible with today's MEMS technology, the integration of a cavity containing a gas species into a wafer with integrated waveguide structures and/or optical resonators is essential. The present description expressly understands a cavity to mean a gas-filled cavity and not an optical cavity such as, for example, a laser resonator. Exemplary proposals for such gas-filled cavities are, for example, in the publication US8385693B2 and the paper by Karlen, Sylvain & Buchs, Gilles & Overstolz, Thomas & Torcheboeuf, Nicolas & Onillon, Emmanuel & Haesler, Jacques & Boiko, D. (2017), "MEMS atomic vapor cells for gyroscope applications", DOI:10.1109/ FCS.2017.8088879.

Chip-integrated cavities are inevitably of small volume; their largest inner diameter can be a few millimeters up to a few centimeters. Due to the manufacturing process, integrated cavities are usually disc-shaped, i.e. their dimensions are determined by a short axis (z) and two long axes perpendicular to it (x, y - spanning the plane of the disc), with the diameter of the cavity in the x, y direction being comparably large and clear are larger than the diameter in the z-direction. The wall of the cavity forms the housing of the gas cell and can be made of different materials. In particular, the enclosure may include windows that are transparent to light, e.g. made of glass.

It is often difficult to obtain a sufficiently strong optical signal by absorption or fluorescence when the light beam only traverses a short distance through the gas. As a remedy, it is known to mirror the inner walls of a gas cell and thus to provide a plurality of crossings between the light entry and light exit windows. This plurality N is usually a small natural number, typically N < 10, and the path length of the light covered in the gas is precisely known and is smaller than N times the largest inner diameter of the gas cell. In addition, the light propagates in these cases as a free beam within the gas, and the fanning out of the light as well as unwanted disturbances due to reflection, scattering, diffraction (e.g. when radiating from an optical fiber) lead to geometric losses of intensity at the exit window that increase with N. It is desirable for the light to be guided as continuously as possible in a waveguide and thus with a controllable mode diameter.

An alternative to a gas cell can in principle be seen in a hollow core waveguide, which consists of a fiber cladding ("cladding") surrounding a cavity along the fiber direction. The cavity may be vacuum or gas composition. The light is guided in the cavity - also: hollow core - whereby the interior of the hollow core can have a reflective, e.g. metallic or optical coating. Hollow-core fibers are suitable for transporting a very wide range of wavelengths and are therefore recommended for optical sensors.

Filling the hollow core with gas evenly can prove to be complicated, for example if the desired gas species is only available at elevated temperature is gaseous or if adsorption of the gas on the inner wall of the fiber or other surfaces of the system is to be expected, which can limit the transmission. For more information see Nikodem's review article, "Laser-Based Trace Gas Detection inside Hollow-Core Fibers: A Review", Materials 2020, 13, 3983; doi:10.3390/ma13183983. In the one shown there 2 refers to the filling of the HCWG from the fiber ends or through a plurality of channels in the fiber cladding. In the first-mentioned cases, the gas-filled hollow-core fibers are wound up into a spool in order to accommodate a light path length (possibly a few meters) in a limited installation space. In contrast, it is striking that the light guide, which is provided with channels, is arranged in a gas cell (which can be opened here) and runs without any bends.

The pamphlet U.S. 7,259,374 B2 describes a hollow-core fiber coil as the gas space in a compact spectrometer for the detection of trace gases. However, the smallest radius of curvature of the hollow-core fiber has a lower limit because a radius that is too small can lead to losses and ultimately to reversal of the beam direction: "In this case, the angle of incidence of the impinging radiation upon the inner wall surface plays a crucial role in determining whether the radiation will continue to go forwards, trapped into stationery resonant reflections or reversing its reflection direction going backwards altogether.” In the publication, the smallest radius of curvature is given as an example of 1 inch = 2.54 cm.

In addition to the usual or internally mirrored hollow-core fibers, in which the light is guided by reflections, anti-resonant hollow-core waveguides (“hollow-core anti-resonant fibers”, HARF) are now also known, on whose fiber cladding inner wall a plurality of thin fiber strands form the hollow core are arranged symmetrically surrounding the HARF. An exemplary and typical arrangement comprises six strands, the centers of which form a regular hexagon in the cross-section of the HARF. The lighting is based here on constructive interference of the modes running in the strands in the central area between the strands and destructive interference outside. The guided light intensity is greatest in the hollow core of the HARF, where it can be brought into direct contact with a gas, among other things.

In the work of Shou-Fei Gao, Ying-Ying Wang, Xiao-Lu Liu, Wei Ding, and Pu Wang, "Bending loss characterization in nodeless hollow-core anti-resonant fiber," Opt. Express 24, 14801-14811 ( 2016) it is investigated how large the intensity losses of a curved HARF can be. The smallest acceptable radius of curvature of a HARF mentioned in this work is 2 cm.

From the pamphlet CN 111257992B it can be seen that the losses due to bending of a HARF can be improved by shifting the hollow core between the strands. For this purpose, the strands are then designed with different thicknesses.

So-called light cages have been known as further hollow-core waveguides for a few years, cf Jain et al., "Hollow Core Light Cage: Trapping Light Behind Bars", ACS Photonics 2019, 6, 3, 649-658 . Light cages are related to HARFs in that the light guidance is also based on the interference principle, whereby the fiber cladding has now been largely opened - more precisely: almost completely removed. In fact, the strands arranged around the hollow core are now largely free; they are only held at a predetermined distance from one another at certain points by connecting elements between the strands. In this case, the connecting elements form a ring structure together with the strands, and a plurality of ring structures is arranged at customary regular intervals along the HCWG in order to fix the strands in their parallel course to one another. The parallel strands are often referred to as the "bars" of the light cage, between which the light is "locked". The structures can be produced, for example, by UV laser structuring of a block of photoresist polymer and subsequent bath in the developer. Obviously, light cages have the advantage of the best possible accessibility of the hollow core (“air core”, from now on referred to as core) for gases from the environment.

The work of Davidson-Marquis et al., "Coherent interaction of atoms with a beam of light confined in a light cage," Light: Science & Applications (2021) 10:114 uses a light cage made on a silicon substrate in a gas cell filled with cesium to generate an EIT and proves that the light-conducting core of the light cage can easily be filled with a sufficiently high density of Cs atoms (approx. 7 × 10 ¹⁰ cm ^-3 ). The efficiency of a light delay path realized with EIT in the light cage is judged to be comparable to a free-beam setup.

Against this background, the object of the invention is to design a gas cell that can be integrated into chip technology in such a way that it enables the light radiated into the gas to have a greater path length than in the prior art.

Summary of the Invention

The object is achieved by a gas cell comprising an enclosed gas in a disk-shaped cavity, the wall of the cavity for light of a predetermined wavelength interval having at least one transparent entrance window and one transparent exit window, and the light passing from an optical entrance at the entrance window to an optical exit at the Exit window is guided through a gas-filled hollow-core waveguide arrangement, characterized in that the hollow-core waveguide arrangement partially includes parallel light cages.

The subclaims specify advantageous configurations.

According to the invention, the light cages, which run parallel in sections, are preferably arranged next to one another in a plane that coincides with the pane plane of the cavity. Each light cage has connecting elements, cantilevered strands and a light-carrying core - known from the prior art - and the greatest distance between the outsides of two strands perpendicular to the core run is defined as the diameter of the light cage. There is at least one adjacent light cage for each light cage, with the center distance of the cores of two adjacent light cages preferably being - i.F. in short: core-core distance - is not greater than twice the diameter of the light cages along the entire length of the parallel course. According to the invention, a light beam passes through adjacent light cages in immediate succession. The total path length of the light between the entry and exit windows corresponds at least to the total length of the light cages and can preferably be set up according to the invention such that it is at least a factor of 10 greater than the largest diameter of the disk-shaped cavity.

character list

• 1 eine beispielhafte Anordnung gerader paralleler Lichtkäfige in einer quaderförmig ausgebildeten Kavität in einem Wafer, wobei das Licht mittels 90°-Winkelspiegeln von je einem Lichtkäfig in den benachbarten Lichtkäfig retroreflektiert wird;
• 2 eine beispielhaft spiralförmige Anordnung von gekrümmten Lichtkäfigen in einer kreiszylinderförmig ausgebildeten Kavität in einem Wafer, wobei das Licht von einer außen liegenden Spiralschleife an die benachbart weiter innen liegende direkt durchgeleitet wird.

The invention is explained in more detail with reference to the following figures. It shows:

• 1 an exemplary arrangement of straight, parallel light cages in a cuboid cavity in a wafer, the light being retro-reflected by means of 90° angled mirrors from one light cage each into the adjacent light cage;
• 2 an exemplary spiral arrangement of curved light cages in a circular-cylindrical cavity in a wafer, wherein the light is passed directly from an outer spiral loop to the adjacent inner one.

Detailed presentation of the invention

In 1 is a first exemplary embodiment of the invention in three sketches - left: oblique view, top right: plan view of the disk plane, bottom right: side view - shown. The gas cell (1) according to the invention is designed as a disc-shaped cuboid with two x and y axes of equal length and a shorter z axis. It also includes a cuboid, disk-shaped cavity that can be produced, for example, by an etching process in silicon. The lateral wall of the cavity is formed from the silicon wafer (11). On the top and bottom of the wafer (11), for example, glass panes (12) are attached using known methods, for example anodically bonded, which thus form the upper and lower wall of the cavity.

Light cages (13) running parallel along one of the long pane axes are arranged within the cavity. The individual light cages (131) are all designed here as straight stretches and comprise self-supporting strands (not shown individually), which are held and fixed in their relative arrangement to one another by support structures (132) that are regularly spaced apart. The support structures (132) can also be connected to one another between two adjacent light cages (131) in such a way that the parallel arrangement of the light cages (13) is ensured. The strand ends of each light cage form the optical inputs or outputs (1311).

Where the silicon wafer (11) forms the lateral wall of the cavity, linear arrays of 90° angled mirrors (112) are embedded in the wall on two opposite sides. A 90° angled mirror (112) consists of two mirror surfaces - here: mirror-coated silicon - which are at right angles to one another. The normals of the two mirror surfaces span the plane of incidence of a light beam in which the light beam is reflected exactly back in the direction of incidence, i.e. in which the 90° angled mirror (112) acts as a retroreflector with a lateral offset of the emitted light beam compared to the incident light beam. In this example, the arrangement of the light cages (13) and the angle mirror arrangements (112) are designed in such a way that the optical inputs or outputs (1311) of each pair of adjacent light cages (131) have a retroreflective angle mirror (112) arranged opposite them . In the context of this description, “jointly opposite” is to be understood as meaning that the light emerging from the optical output (1311) of the first light cage (13) has the retroreflector property of one of the Angle mirror (112) is directed precisely into the optical entrance (1311) of the second light cage (13). The arrangements of the corner mirrors (112) in the opposite sides of the wall are offset from one another perpendicular to the direction of light guidance of the light cages (131) by a neighboring distance of the light cages (131), so that a light beam follows a meandering path from the entrance window (111) to the exit window (111). puts back.

The parallel arrangement of straight light cages (13) with 90° angled mirrors (112) is advantageous for cuboid cavities, with the light cages (131) preferably being aligned along the longest rectangular axis of the pane plane in order to minimize the number of angled mirrors (112) required.

In the embodiment of 1 the gas cell (1) can comprise a cavity with dimensions of 10×10×1.4 mm ³ and have, for example, 24 light cages (131) with a diameter of 0.2 mm and a core-core distance of 0.4 mm. The total length of the light cage arrangement (13) is then approximately 24 cm, approximately 17 times the largest diameter of the disk-shaped cavity. It should be noted that in some applications the light should come in perpendicular to the plane of the pane and should also exit the entrance or exit window (111) perpendicular to the plane of the pane. A free light beam would then only have a light path of approx. 1.4 mm in the gas. However, it is well known that the incident light can be directed into the pane plane by means of a first mirror behind the entrance window (111) and, after passing through the cavity, directed to the exit window (111) with a second mirror (see Chutani, R., Maurice , V., Passilly, N. et al "Laser light routing in an elongated micromachined vapor cell with diffraction gratings for atomic clock applications", Scientific Reports 5, 14001 (2015).https://doi.org/10.1038/srep14001 ). The multiple guidance of light through the cavity by means of a partially parallel arrangement of light cages (13) is added here according to the invention.

In the first exemplary embodiment, the light covers short path sections when passing from one light cage (131) to the next as a free beam. This results in a geometric loss of intensity, which is caused by the beam fanning out as it leaves a light cage (131). An estimate taking into account the overlap integral of the modes leads to losses of about -0.3 dB per transition from one light cage (131) to the next. By advantageously arranging collimating lenses (not shown) at the inputs and/or outputs (1311) of the light cages (131), these losses can be further reduced, as has already been shown for optical fibers (I. Weiss and D. M. Marom, " Direct 3D nanoprinting on fiber tip of collimating lens and OAM mode converter in one compound element," in Optical Fiber Communication Conference, OSA Technical Digest (online) (Optical Society of America, 2016), paper Th3E.2. https://doi .org/10.1364/OFC.2016.Th3E.2). Such lenses can be added directly onto the light cage ends (1311) by 3D nanoprinting, for example.

In a second exemplary embodiment of the invention, it is assumed that the cavity produced in a silicon wafer is circular-cylindrical, e.g.

In 2 a gas cell (2) is shown in an oblique view (left) and top view (right), which, as in the first example, is formed from a silicon wafer (21) with a cavity and two glass panes (22) bonded on. The light cage assembly (23) in the cavity is now spiral shaped, with a single spiral loop, ie a complete 2π orbit, now being considered a portion of the light cage assembly (23) or also a single curved light cage (231). As before, each light cage (231) has strands (not shown individually), support structures (232), and optical inputs and outputs. Each light cage (231) runs parallel to at least one neighboring light cage (231), the first neighbor being on the outside and the second on the inside. The light cages (231) are formed with a first radius of curvature at the beginning and with a second radius of curvature smaller than the first radius of curvature at the end, with two adjacent light cages (231) being connected at the point at which the second radius of curvature of the outer light cage (231 ) and the first radius of curvature of the inner light cage (231) match. At the same time, the optical output of the outer spiral loop light cage (231) coincides with the optical input of the inner spiral loop light cage. In this way, a ray of light is "transferred" directly from one light cage (231) to its neighboring one, ie the light propagates through the spiral of light cages (23) without leaving it. Consequently, the spiral (23) has a total of only two optical inputs or outputs (2311). The entry window and exit window of the gas cell (2) are to be provided there, possibly with a deflection mirror for deflecting the light into the pane plane

To the inventors' knowledge, no curved light cages (231) have been proposed or implemented in the prior art. Since they behave very similarly to HARFs in terms of light guidance, the published findings on acceptable radii of curvature of HARFs can be transferred to spiral loops (231). Accordingly, it is advantageous if the cavity of 2 , in which the spiral (23) is arranged, has at least a disc diameter of 4 cm. Then the radius of curvature of the outermost spiral loop (231) is about 2 cm and shows small losses in intensity. For light cages (231) located further inwards, radii of curvature and lengths decrease continuously, but the provision of 4-5 spiral loops ensures that the path length of the guided light can be increased to at least 10 times the largest diameter of the cavity. It should be noted that 2 shows only three spiral loops (231) because the outermost one has been omitted for clarity.

It can be seen that in contrast to the preferred structurally identical straight light cages (131) of 1 the spiral loop-shaped light cages (231) of 2 cannot be structurally identical. Rather, each spiral loop (231) has its own courses of radii of curvature and is thus structurally fixed with regard to its position within the parallel arrangement of the spiral loops (23). At the same time, it is possible and advantageous that the individual strands of the curved light cages (231) can also be varied in the course of a spiral loop. For example, the thickness of the strands can change continuously, so that the light-guiding core area surrounded by the strands is no longer exactly in the center of the light cage (231), as is shown in FIG CN 111257992B for HARFs is proposed. The previously tested production of light cages (131, 231) by 3D structuring by means of UV laser irradiation of photoresist blocks and subsequent development is also easily suitable for such more complex, variably running (networked) structures with high precision. A suitable photoresist is, for example, a diacrylate-based photoresist such as IP-Dip, from which the light cages (131, 231) can be formed under UV illumination.

Depending on the height of the disk-shaped cavity, it can be possible and advantageous for the arrangement of the light cages (13, 23) in a first disk plane to be followed by at least one second arrangement (13, 23) in a second disk plane that is parallel to the first disk plane. This is particularly easy to do in the case of the meandering light guide 1 , by arranging an additional angle mirror at the end of the light path of the first arrangement (13) - at the optical output (111), which retroreflects the light beam offset by a distance in the z-direction and thus the second - preferably then identical - arrangement (13 ) supplies. The guided light then "meanders" in the short z-axis, offset back the same way. Other pane levels can also be used if there is enough space in the cavity.

It is already common and very advantageous for the gas cells (1, 2) according to the invention if the walls of the cavities (11, 12, 21, 22) are equipped with means for controlling the temperature in the gas cell in order to keep this temperature in a range above the ambient temperature and to stabilize the gas pressure. The gas pressure must usually be kept constant, even if the chip - the entire device - should heat up during operation, for example due to the waste heat from chip components, e.g. an integrated laser. In the event of such heating, the heating output of heating coils arranged, for example, on the glass panes (12, 22) must be reduced accordingly. For this purpose, the means for controlling the temperature preferably also include temperature and/or pressure sensors, which are arranged on the wall and continuously monitor the interior of the cavity. The electrical sensor measurement data can be supplied to an evaluation device, which is preferably integrated into the chip that also includes the gas cell. However, the evaluation device and a control device for energizing e.g. heating coils are not necessarily integrated into the gas cells (1, 2) themselves, but can also be connected as external components.

In particular, it is preferred if the gas cell can be filled with at least one alkali metal vapor, with heating coils providing the elevated temperature for evaporating the alkali metal and being used to stabilize the alkali metal vapor pressure.

QUOTES INCLUDED IN DESCRIPTION

This list of documents cited by the applicant was generated automatically and is included solely for the better information of the reader. The list is not part of the German patent or utility model application. The DPMA assumes no liability for any errors or omissions.

Patent Literature Cited

• US 8385693 B2 [0004]
• US 7259374 B2 [0009]
• CN 111257992B [0012, 0029]

Non-patent Literature Cited

• Jain et al., "Hollow Core Light Cage: Trapping Light Behind Bars", ACS Photonics 2019, 6, 3, 649-658 [0013]
• Davidson-Marquis et al., "Coherent interaction of atoms with a beam of light confined in a light cage", Light: Science & Applications (2021) 10:114 [0014]

Claims (10)

Gas cell (1, 2) comprising an enclosed gas in a disc-shaped cavity, the wall of the cavity (11, 12, 21, 22) having at least one transparent entrance window and one transparent exit window for light of a predetermined wavelength interval, and the light from one optical input (111, 2311) at the entry window is led to an optical output (111, 2311) at the exit window through a gas-filled hollow-core waveguide arrangement (13, 23), characterized in that the hollow-core waveguide arrangement ( 13, 23) partially parallel light cages (131, 231). gas cell after claim 1 , characterized in that the light cages (131, 231) are arranged next to one another in a plane which coincides with the plane of the pane of the cavity. Gas cell according to one of the preceding claims, characterized in that the core-core distance between two adjacent light cages (131, 231) in the hollow-core waveguide arrangement (13, 23) is not greater than twice the cross-sectional diameter of the light cages (131, 231). Gas cell according to one of the preceding claims, characterized in that the total length of the light cages (131, 231) is greater by at least a factor of 10 than the largest diameter of the disk-shaped cavity. Gas cell according to one of the preceding claims, characterized in that the light cages (131) are designed as straight sections, with a retro-reflective corner mirror (112) being arranged opposite the optical inputs or outputs (1311) of each pair of adjacent light cages (131). . gas cell after claim 5 , characterized in that collimating lenses are arranged at the optical inputs and/or outputs (1311) of the light cages (131). Gas cell after one of Claims 1 until 4 , characterized in that the light cages (231) are designed as spiral loops with a first radius of curvature at the beginning and a second radius of curvature smaller than the first radius of curvature at the end, two adjacent light cages (231) being connected at the point at which the second radius of curvature of the outer light cage (231) and the first radius of curvature of the inner light cage (231) match. Gas cell according to one of the preceding claims, characterized in that the arrangement of the light cages (13, 23) in a first pane plane is followed by at least one second arrangement (13, 23) in a second pane plane parallel to the first pane plane. Gas cell according to one of the preceding claims, characterized in that the light cages (131, 231) are formed from a polymer which is crosslinked under UV illumination. Gas cell according to one of the preceding claims, characterized in that the wall of the cavity (11, 12, 21, 22) has means for regulating the temperature in the gas cell (1, 2).

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